1. Field of the Invention
The present invention relates to an objective lens driving device for an optical head which is mounted on a disk driving apparatus for an optical disk, an optical magnetic disk, etc., and more particularly, to an objective lens driving device for an optical head employing a lens having high numerical aperture which requires adjustment of a tilt angle.
2. Description of the Related Art
In an optical disk driving apparatus, an art of correcting position of an objective lens of an optical head by displacing the lens respectively in a focus direction of light and in a radial direction (a track direction) of a disk which intersects with data rows on the disk has been heretofore known.
As a conventional structure for conducting corrections in the focus direction and in the track direction, there has been known such a structure that providing coils and magnets respectively on side faces of a lens holder which holds an objective lens and outside of the lens holder, the lens holder is supported by wires so as to be displaced in the focus direction and in the track direction, and control electric currents are supplied to the afore the coils by way of these wires. This structure has permitted a compact and low-cost positional correction to be realized.
Generally, in a position correcting mechanism having the above described structure, four wires in total consisting of a pair of wires for supplying the electric currents for focus correction and another pair of wires for supplying the electric currents for track correction are used. Such a supporting system of the lens holder by means of the four wires can conduct the support and the positional correction of the objective lens in a relatively stable manner (See JP-A-2000-322752).
By the way, high densifying technique of the optical disk has been recently developed, and recording density of four to five times of that of a CD (a compact disk) has been realized in a DVD (a digital versatile disk). In order to realize a disk driving apparatus employing such a high densified optical disk, it has been necessary to increase numerical aperture (NA) of the objective lens for the purpose of increasing amounts of an emitting light, or to shorten a wavelength of a semiconductor laser (LD).
Among the above described measures, by increasing the numerical aperture of the objecting lens, a light condensing spot can be made small in diameter, and even more minute bits on the optical disk can be advantageously reproduced. On the other hand, in case where an inclination of angle occurs between the objective lens and the optical disk due to deflection or oscillation of the optical disk, this will be a factor of increasing coma aberration of the light condensing spot and deteriorating a shape of the light condensing spot, which results in such an inconvenience that noises are included in a pickup signal. For this reason, in order to realize the high numerical aperture of the objective lens, it is necessary, in addition to the positional correction in the above described focus direction and track direction, to conduct correction of the tilt angle of the objective lens according to an inclination of the disk in a radial direction, so as not to generate the inclination of angle between the objective lens and the optical disk.
As a conventional optical head in which corrections in three axial directions, namely, the focus direction, track direction and tilt angle direction are possible, there has been such a device, as disclosed in JP-A-2000-285490, for example, that a mechanism for conducting the correction in the focus direction and the track direction is provided on a base plate which can be displaced in the tilt angle direction, and this base plate is driven to rotate in the tilt angle direction by a moving magnet method (a driving method by means of fixed coils and movable magnets) or the like.
More specifically, in an “Objective Lens Driving Device” according to JP-A-2000-285490, electric currents are supplied so that directions of driving forces arising in focusing driving coils in four driving coil blocks may be opposite to each other in a radial direction of the disk with respect to a center of the objective lens, thereby to obtain the driving force in a rotation direction (a radial tilt direction) making a tangential direction of the disk as a rotation axis.